The present invention concerns the filtering of a digital signal, such as the transformation of a digital signal into frequency sub-band signals.
Many digital filtering methods and devices are known. By way of example, digital signal analysis filterings and corresponding synthesis filterings are envisaged here.
These filterings are generally subsystems integrated into the coding and/or decoding systems. They often require a large amount of random access memory or buffer memory space, for storing the data in the course of processing.
However, in practice, the size of the memory means are often less than the size which would be necessary for storing an entire set of data, for example a digital image. It is therefore necessary to xe2x80x9cdividexe2x80x9d the signal into blocks and to process the blocks one after the other.
However, between an analysis and the corresponding synthesis of a signal, other processings, such as quantization or entropic coding, are generally applied to said signal. These processings, combined with the processings by blocks, cause degradation in the reconstructed signal.
The present invention provides a method and device for transforming a digital signal which processes the signals by blocks, whilst limiting the degradation in the reconstructed signal.
To this end, the invention proposes a method for the analysis filtering of an original digital signal including original samples representing physical quantities, original samples of the digital signal being transformed by successive calculation steps into high and low frequency output samples, any sample calculated at a given step being calculated by a predetermined function of original samples, and/or previously calculated samples, the samples being ordered by increasing rank,
characterised in that:
the signal is processed by successive input blocks of samples, calculations made on an input block under consideration taking into account only the original or calculated samples belonging to the input block under consideration,
the input block under consideration and the following input block overlap over a predetermined number of original samples,
output blocks are formed, each output block corresponding respectively to an input block,
two consecutive output blocks overlap over output samples having the same ranks as the original samples on which the input blocks corresponding to the output blocks under consideration overlap.
By virtue of the invention, the data are coded and then decoded by blocks, whilst limiting the discontinuities at the block limits. In addition, any block is coded and decoded independently of other blocks.
Moreover, the inventors have found that the buffer memory occupation by the data in the course of processing is optimised. Thus complex filterings can be integrated into a large number of apparatuses, without these requiring very large memories.
According to a preferred embodiment, the blocks are formed in the following manner:
the start limit of the input block under consideration is formed between a first original sample and a first output sample, passing successively, from a previous sample to a following sample calculated as a function of the previous sample, the following sample having a rank equal to or greater than the previous sample,
the end limit of the input block under consideration is formed between a second original sample and a second output sample, passing successively from a previous sample to a following sample, calculated as a function of the previous sample, the following sample having a rank equal to or less than the previous sample,
the end limit of the input block under consideration and the start limit of the following input block are such that there does not exist any compatibility sample having a rank strictly less than the rank of the sample belonging to the start limit of the following input block and to the same row as the compatibility sample, and at the same time a rank strictly greater than the rank of the sample belonging to the end limit of the input block under consideration and to the same row as the compatibility sample.
According to a preferred characteristic, two consecutive output blocks overlap over a single output sample. Thus the memory occupation of the data in the course of processing is minimised.
Preferably, two consecutive output blocks overlap over a single output sample, which is a low-frequency output sample. Thus the discontinuities on reconstruction are limited.
The invention also concerns a method of coding a digital signal including original samples representing physical quantities, the coding including:
an analysis filtering effected according to the above method, in order to form filtered samples,
a quantization of the previously filtered samples, and
an entropic coding of the previously quantized samples.
The coding method has advantages similar to these of the filtering method.
According to a preferred characteristic of the coding method, the same quantization, with the same parameters, is used for two successive blocks, at least for the samples situated in the overlap part of the two blocks under consideration. Thus the samples present in two blocks are coded and decoded at the same value.
Preferably, the quantization is a scalar quantization and the same scalar quantization, with the same quantization step, is used for two successive blocks, at least for the samples situated in the overlap part of the two blocks under consideration.
The discontinuity effect at the limits of the blocks is reduced to the maximum possible extent.
According to preferred characteristics, the digital signal is an image signal and the original samples are rows or columns of the image. The invention advantageously applies to an image signal, which generally requires a large amount of memory space. This memory space is reduced by virtue of the invention.
Correlatively, the invention proposes analysis filtering and coding devices, which have means adapted to implement the above characteristics.
The invention also concerns a digital apparatus including the above transformation device or means of implementing the transformation method. The advantages of the device and of the digital apparatus are identical to those previously disclosed.
An information storage means, which can be read by a computer or by a microprocessor, integrated or not into the device, possibly removable, stores a program implementing the filtering method.